Recovery of sodium sulfate



Patented Sept. 5, 1950 RECOVERY OF SODIUM SULFATE Horace James Hegan, Coventry, England, as-

signor to Courtaulds Limited, London, England,

a British company No Drawing. Application November 3, 1947, Se-

rial No. 783,846. In Great Britain December Claims.

This invention relates to the recovery of chemical substances from viscose rayon spinning bath liquors containing sulphuric acid, sodium sulphate and also zinc sulphate in a proportion of less than 2 per cent. It also includes such recovery from liquors which contain additional compounds such as magnesium sulphate.

In the specification of my pending application Serial No. 664,441, filed April 23, 1946, I have described a process for the recovery of chemical products from a viscose rayon spinning bath containing sulphuric acid, sodium sulphate and zinc sulphate in which water is removed by evaporation', whilst maintaining the temperature of the liquor abov 32 centigrade, in order to precipitate crystals of anhydrous sodium sulphate associated with complex salts of sodium sulphate, sulphuric acid and zinc sulphate, separating the crystals from the mother liquor at a temperature greater than 32 centigrade and then submitting the separated crystals to counter-current washing with an aqueous solution of sodium sulphate at a temperature substantially greater than 32 centigrade, the proportion of water in the washing solution being insufiicient to dissolve all of the sodium sulphate contained in the crystals, but sufiicient to decompose the complex salts and to dissolve all sulphuric acid and zinc sulphate present.

It is the object of the present invention to take advantage of investigations which show that the results obtained from such a process are greatly enhanced and the process is more economic than previous processes when certain limits are observed as to the acid concentrations of the precipitation liquor and of the washing solutions.

According to the present invention a process for the recovery of chemical substances from viscose rayon spinning baths containing sulphuric acid, sodium sulphate and also zinc sulphate in a proportion ofless than 2 per cent includes the steps of removing water by evoporation from the spinning bath liquor and if necessary adding acid to the bath liquor until the total proportion by weight of sodium sulphate, calculated as NazSO4, present in the liquor and as precipitated crystals exceeds 30 per cent of the weight of the concentrated liquor and the percentage by weight of acid in the concentrated liquor exceeds 1'? per cent of the weight of the concentrated liquor, cooling the concentrated liquor so obtained to below 32 centigrade to precipitate complex salts of sodium sulphate, sulphuric acid and zinc sulphate, separating the crystals from the mother liquor and decomposing the precipitated complex salts to their three constituent salts by treatment with washing liquor containing sulphuric acid and sodium sulphate under such conditions that when the acid concentration of the wash liquor in contact with the crystals is above 17 per cent by weight, the temperature of the washing liquor is maintained below 32 centigrade and when the acid concentration of the wash liquor in contact with the crystals is below 17 per cent by weight, the temperature of the wash liquor is maintained above 32 centigrade.

In the step of removing water by evaporation, the evaporation is preferably carried out until the weight of sodium sulphate present exceeds 35 per cent and the acid concentration exceeds 25 per cent. In order to enable this acid concentration to be reached, it may be necessary to add sulphuric acid to the spinning bath before or during the evaporation.

In carrying out the step of decomposing the precipitated complex salts considerable advantages are obtained by countercurrent or percolative washing in which the acid concentration decreases as the decomposition of the complex salts progresses and the temperature of the washing liquor is increased at and beyond the stage at which its acid content reaches 1'7 per cent. For example in the first part of the decomposing stage the acid concentration of the mother liquor leaving the crystals will be of the order of 25 per cent or higher and the temperature preferably should not exceed 20 centigrade. As the washing progresses the acid concentration of the wash liquor in contact with the crystals decreases to below that of 1'7 per cent and the temperature can then be increased to above 32 centigrade and may be as high as 50 centigrade or even higher.

Nevertheless according to one method of carrying out the process of the present invention the separated complex sodium sulphate, sulphuric acid, and zinc sulphate salts particularly if the sulphuric acid and zinc sulphate contents are not very high may be subjected to a single step treatment with spinning bath or channel liquor evaporated to a high content of sodium sulphate if suilicient of these solutions be available when the decomposition and solution will take place in one stage. Channel liquor is the name given to the drip liquor from the formed filaments. Care must be taken that the acid concentration does not rise about 17 per cent because acid solutions containing above 17 per cent sulphuric acid will not completely decompose the complex salts; v

The mother liquor after the separation of the complex crystals, for example by decantation or centrifuging, and after adjustment if necessary, may be returned to the spinning bath; alternatively as its acid content is high a proportion or all of it may be added to the spinning bath about to be evaporated so as to increase the acid content.

The decomposing solutions may be made up from the. spinning bath liquor itself or from evaporated spinning bath liquor, or from channel liquor or from a mixture of any of these.

The solutions resulting from the decomposition step, in particular those of high acid content for example of about or abovel'lper cent, can be added to the spinning bathor can be added to the liquor in the evaporator for the purpose of adjusting the acid content of the evaporated liquor as required. Adjustment of the acid content of the liquor in the evaporator can also be made by the addition of concentrated sulphuric acid in quantities less than the equivalent of anhydrous sodium sulphate removed.

The solid residue remaining from the decomposing steps is'largely anhydrous sodium sulphate. In the event of a practically pure acid-free and 'zinc free-anhydrous sodium sulphate being required, crystals which have been treated with acid washing solution with an acid content below 1'? per cent sulphuric acid at a temperature above 32 cen-tigrade and containing only a small proportion of acid and zinc sulphate are washed with water at 32 centigrade or above, or with a saturated solution of sodium sulphate at 32 centigrade or above.

Information which will assist in the determination of the optimum temperatures and acid concentrations to be employed in carrying out the process of the present invention is contained in Solubilities of Inorganic and Metal Organic Compounds by Seidell (D. Van Nostrand Company Inc.)' third'ed'iti'on, volume 1, pages 1303- 130"]. It haszbeen found that simple test will readily show the optimum figures when small modifications are necessary for example due to the presence in the spinning bath liquor of substances additional to sulphuric acid and sodium sulphate. No substantial modification is necessary due to the presence of zinc sulphate provided it is present in a quantity of less than 2 per cent.

The percentag proportions referred to in this specification are based on the weights of H2804, Na'zSU; and ZIlSOa. present in the solutions.

The process of the present invention is illustrated by the following specific examples:

Example 1 (a) 2000 grams of spinning bath containing '9 per cent H2304, 0.8 per cent ZnSOr, 25 per cent Na2SO4 and 64.2 per cent H2O wer mixed with 150 grams of concentrated sulphuric acid grams of mother liquor obtained from a vious batch and containing 28 per cent lit-S04, 27 per cent Na2SO4, 1.5 per cent ZnSO4 and per cent 20. The mixture was evap orated to 70 centigrade until 1000 grams of water had been removed. At this point crystallisation was about to begin and th hot evaporated solution contained 29.4 per cent H2804, 37.5 per cent Nae-S02, 1.4 percent ZnSO4 and 31.6 per cent H2O.

The solution was then cooled to 21 centigrade and the precipitated crystals separated by drainon a coarse filter.

830 grams of filtrate were removed containing 4 29 per cent H2804, 1.9 per cent ZDSO4 and 26 per cent Na2S04.

(221) The crystals were then washed by percolation, firstly with those wash liquors obtained from a previous batch. These wash liquors cont'ained from l8'per cent down to 16 per cent H2804 and the temperatures increased from 15 centigrade for the 18 per cent-H2SO4 to 40 centigrade for the 16 per cent H2804. Particular care was taken to ensur that the temperature was always above 32 centigrade when the acid concentration in the wash liquor was below 17 per cent. The percolation was carried out with a slow stirring of the magma in order to prevent the formation of an impermeable layer at the level of breakdownof the NaHSO4 in the crystals, which causes a fall in temperature of about 5 centigrade.

The filtrate which drained from the crystals during this percolation process gradually decreased in acid concentration from 29 per cent to 18 per cent while the ZnSO4 decreased from 1.9 per cent to 1.1 per cent and the total quantity of washings obtained was 1400' grams. Part of this was added to some of the mother liquor to adjust the strength of H280; and ZnSOr for use in the evaporation of the next batch and the remainder of the mother liquor and washings was returned to the spinning bath.

(122) The crystals were then washed by percolation with evaporated overflow spinning bath liquors containing 12 per cent H2SO4, 33 per cent Na2SO4, 1 per cent-Zn'SO4 and 54' per cent H2O, at a temperature of from 45 Centigrade to 70 centigrade. 1200 grams of this liquor were used, the filtrate amounting to 1460 grams which gradually decreased in acid concentration from 18 per cent to 1'7 per cent and then to 15 per cent. This filtrate was used for washing the next batch as described under (b1). At this stage the complex acid salts were completely broken down and the crystals contained'5 per cent H2SO4 and 0.10 per cent-ZnSO4. They were finally washed by percolation with a minimum of hot water to re move the adhering acid" solution, the filtrate being reserved for treatment of the next batch as under (bl) and (b2). 2'75 grams of anhydrous sodium sulphate free from acid and zinc salts were obtained.

The quantity of acid added to adjust the concentration of the solution in the evaporator was less than the equivalent of the sodium sulphate recovered.

Example 2 (a) 1330 grams of spinning bath containing 10 per cent H2804, 24 per cent Na2SO4 and 0.6 per cent ZnSO4 were mixed with 4000 grams of wash liquor from a previous batch, containing 18 per cent H2504, 35' per cent Na2SO4 and 1.0 per cent ZnSOr and the mixture was evaporated to 3460 grams with loss'of nearly 40 per cent of its weight of water. The mag-ma of mother liquor and crystals was cooled to 20 centigrade with further precipitation of crystals. The mother liquor was separated by decantation and filtration. It weighed 1670 grams and contained about 29 per cent H2SO4. This liquor was returned to the spinning bath.

(b) The crystals which weighed approximately 1800 grams and contained about 21 per cent H2804 were treated with 3240 grams of concentrated channel liquor at 50 centigrade, this liquor being'obtainedby evaporation of channel liquor un-til saturation with-'Na2SO4 was reached.

It contained about per cent H2804, 34 per cent NazSO4 and 0.9 per cent ZnSO4.

The treatment was continued by agitation at 50 centigrade until the H2804, Na2SO4, ZnS04 complex was completely decomposed. The wash liquor was then removed by filtration and amounted to approximately 4000 grams and contained about 18 per cent H2804. It was then available for addition to the next batch of spinning bath to be run into the evaporator.

The decomposed crystals amounting to about 1000 grams and containing only small proportions of H2804 and ZnS04 were washedfree from acid and ZnSO4 by hot saturated sodium sulphate solution.

What I claim is:

l. A process for the recovery of chemical substances from viscose rayon spinning baths containing sulphuric acid, sodium sulphate and zinc sulphate, said zinc sulphate being present in an amount greater than 0 and less than about 2 per cent, which comprises removingwater by evaporation from the spinning bath liquor until the total proportion by weight of sodium sulphate calculated as Na2SO4 present in the liquor and as precipitated crystals exceeds 30 percent of the weight of the concentrated liquor and adjusting the sulphuric acid content of the concentrated liquor until it is in excess of 17 per cent by weight temperature below 32 centigrade with aqueous satunated sodium sulphate liquors containing sulphuric acid in a concentration of at least 1'7 per cent by weight, and then washing the crystals at a temperature above 32 centigrade with aqueous saturated sodium sulphate liquors containing sulphuric acid in a concentration greater than 0 per cent but below 17 per cent by weight whereby the complex sodium sulphates present in the crystals are converted into anhydrous sodium sulphate substantially free from acid and zinc salts.

2. A process .as claimed in claim 1 wherein the decomposition is efiected by counter-current washing in which the acid concentration decreases as the decomposition of the complex salts progresses, and the temperature of the washing liquor is increased at and beyond the stage at which its acid content reaches 17 per cent.

3. A process as claimed in claim 1 wherein the evaporation is carried out until the weight of sodium sulphate present exceeds 35 per cent.

4. A process as claimed in claim 1 wherein the evaporation is carried out until the acid concentration exceeds 25 per cent.

5. A process as claimed in claim 1 wherein sulphuric acid is added to the spinning liquor before the evaporation step.

HORACE JAMES HEGAN.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number Name Date 957,417 Glaser et al. -l May 10, 1910 1,519,880 Specketer et al. Dec. 16, 1924 2,233,004 Ebert l Apr. 1'7, 1945 Certificate of Correction Patent N 0. 2,521,367 7 September 5, 1950 HORACE JAMES HEGAN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 52, for the Word about read above; column 4, line 74, before channel insert the;

and that the said Letters Patent should be read as corrected'above, so that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 28th day of November, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

1. A PROCESS FOR THE RECOVERY OF CHEMICAL SUBSTANCES FROM VISCOSE RAYON SPINNING BATHS CONTAINED SULPHURIC ACID, SODIUM SULPHATE AND ZINC SULPHATE, SAID ZINC SULPHATE BEING PRESENT IN AN AMOUNT GREATER THAN 0 AND LESS THAN ABOUT 2 PER CENT, WHICH COMPRISES REMOVING WATER BY EVAPORATION FROM THE SPINNING BATH LIQUOR UNTIL THE TOTAL PROPORTION BY WEIGHT OF SODIUM SULPHATE CALCULATED AS NA2SO4 PRESENT IN THE LIQUOR AND AS PRECIPITATED CRYSTALS EXCEEDS 30 PER CENT OF THE WEIGHT OF THE CONCENTRATED LIQUOR AND ADJUSTING THE SULPHURIC ACID CONTENT OF THE CONCENTRATED LIQUOR UNTIL IT IS IN EXCESS OF 17 PER CENT BY WEIGHT OF THE WEIGHT OF THE CONCENTRATED LIQUOR, COOLING THE CONCENTRATED LIQUOR CONTAINING MORE THAN 30 PER CENT NA2SO4 AND MORE THAN 17 PER CENT H2SO4 TO BELOW 32* CENTIGRADE WHEREBY COMPLEX SALTS OF SODIUM SULPHATE, SULPHURIC ACID AND ZINC SULPHATE ARE PRECIPITATED, SEPARATING THE PRECIPITATED CRYSTALS FROM THE MOTHER LIQUOR AND DECOMPOSING THE PRECIPITATED CRYSTALS TO THEIR CONSTITUENT SALTS BY FIRST REDUCING THE ACID CONTENT OF THE CRYSTALS TO 17 PER CENT BY WEIGHT BY WASHING THEM AT A TEMPERATURE BELOW 32* CENTIGRADE WITH AQUEOUS SATURATED SODIUM SULPHATE LIQUORS CONTAINING SULPHURIC ACID IN A CONCENTRATION OF AT LEAST 17 PER CENT BY WEIGHT, AND THEN WASHING THE CRYSTALS AT A TEMPERATURE ABOVE 32* CENTIGRADE WITH AQUEOUS SATURATED SODIUM SULPHATE LIQUORS CONTAINING SULPHURIC ACID IN A CONCENTRATION GREATER THAN 0 PER CENT BUT BELOW 17 PER CENT BY WEIGHT WHEREBY THE COMPLEX SODIUM SULPHATES PRESENT IN THE CRYSTALS ARE CONVERTED INTO ANHYDROUS SODIUM SULPHATE SUBSTANTIALLY FREE FROM ACID AND ZINC SALTS. 